A surface acoustic wave device (hereinafter referred to as a SAW device) is widely used as a filter or a resonator in radio apparatuses of mobile communication terminals because of its small size and low weight. Since cellular phone systems use higher frequencies, those filters used in the radio apparatuses of mobile communication terminals need to work in the high frequency range between 800 MHz and several GHz, among others. Those filters are required to have low loss in the pass-band, highly attenuate signals in the rejection band, and yet, have steep filter characteristics.
In general, application of a voltage to an inter-digital transducer (IDT), of which finger-electrodes formed on a piezoelectric-substrate mesh with each other, produces a surface acoustic wave (SAW) propagating on the substrate. The SAW device excites and receives this SAW, thereby achieving the filter characteristics needed. The electrical characteristics of the SAW device are based mainly on a shape and a structure of the IDT electrodes, as well as the propagation characteristics of the SAW propagating on the piezoelectric substrate. For instance, a SAW resonator, which is one of the SAW devices, has the following relation:v=2·p·f  equation (1)where p=pitch of the finger-electrodes of the IDT electrodes, v=phase velocity, namely, propagation speed of the SAW, and f=resonance frequency.
Use of a SAW device at a high frequency band increases resonance frequency f, and if propagation speed v is constant, pitch p of IDT finger-electrodes must be reduced. This means that the pattern width of the IDT electrodes should be extremely narrow, which results in lowering the manufacturing yield rate of the SAW devices. To avoid the lowering of the yield rate, a piezoelectric substrate allowing the SAW to propagate at a higher speed (v) is required. To obtain the characteristic of low loss, it is necessary to reduce propagation loss of the SAW and resistance of the IDT electrode as much as possible.
To obtain a SAW device, such as a SAW filter and a SAW resonator of low loss at a high frequency band, the foregoing points must be taken into account. The following two substrates have been thus widely used to obtain the SAW devices. A substrate made of single crystal LiTaO3 (hereinafter referred to as LT single-crystal) has undergone a 36° rotation and a Y-cut. The SAW propagates in the X direction of this resultant substrate, and this substrate is called a 36° Y-X LiTaO3 substrate (hereinafter referred to as a LT36° substrate). A substrate made of single crystal LiNbO3 (hereinafter referred to as LN single-crystal) has undergone a 64° rotation and a Y-cut. The SAW propagates in the X direction of this resultant substrate, and this substrate is called a 64° Y-X LiNbO3 substrate (hereinafter referred to a LN64° substrate). Use of the LT36° substrate and the LN64° substrate allows the SAW device to use a leaky surface acoustic wave (LSAW) which propagates on the substrate while radiating a bulk wave within the substrate.
The LSAW excited by those substrates has a feature of high phase velocity, i.e., propagation speed. Further, when the mass-load effect of the IDT electrode can be neglected, in other words, the film of the IDT electrode is thinner than the wavelength of LSAW to be propagated, the LSAW scarcely radiates the bulk wave. As a result, propagation loss due to the bulk wave can be substantially reduced. Thus, the LT36° substrate and the LN64° substrate are fitted to form SAW filters and SAW resonators of high-frequency and low loss. This is the reason why those two substrates have been widely used.
However, use of thosee substrates in SAW filters or SAW resonators at a frequency range between 800 MHz–several GHz shortens the wavelength of the SAW, so that the film thickness of the IDT electrode becomes as thick as several %–10-odd % of the wavelength. Thus the mass-load effect of the IDT electrode can no longer be neglected. As a result, the propagation loss produced by propagating the LSAW cannot be neglected.
To overcome this problem, use of a substrate with a larger cut-angle is effective for substantial reduction of propagation loss. This idea is disclosed in Japanese Patent Application Non-Examined Publication No. H09-167936. According to this publication, the cut-angle of the substrate, which minimizes the propagation loss of the LSAW propagating on the LT single-crystal and LN single-crystal, varies in response to normalized film-thickness h/λ of the IDT electrode, where h=film thickness of the electrode, and λ=wavelength of the SAW. In the case of the LT single-crystal, when the film thickness of the IDT electrode becomes 0.03–0.15 of the wavelength of the LSAW (normalized film thickness h/λ is 3%–15%), a shift of the cut-angle from 36° to 39°–46° can almost eliminate the propagation loss. In the same manner, in the case of the LN single-crystal, when the film thickness of the IDT electrode becomes 0.03–0.15 of the wavelength of the LSAW (normalized film thickness h/λ is 3%–15%), a shift of the cut-angle from 64° to a greater angle, such as 66°–74°, can reduce the propagation loss to almost 0 (zero).
However, SAW filters and SAW resonators with different frequency characteristics have different pitches of the IDT electrodes, in general. Thus, if SAW filters or SAW resonators having different frequency characteristics with a flat film-thickness are manufactured, each one of the respective filters or resonators has its own optimum normalized film-thickness h/λ different from the rest. As a result, a structure of two filters in one chip, where two SAW filters having different frequencies are formed on one chip, encounters the following problem. In general, the film thickness of the IDT electrode is flat within one chip. Thus, it is difficult for the two SAW filters to reduce their propagation losses almost to 0 (zero) simultaneously. It is necessary to prepare an optimum film-thickness for each SAW filter within one chip to achieve this goal (propagation loss≈0). However, this goal makes the manufacturing steps complicated, and it is difficult to practically achieve this goal on the manufacturing floor.
Japanese Patent Application Non-Examined Publication No. H05-183380 discloses a method of attenuating signals outside the pass-band, and this method is expected to attenuate the signals to a greater degree than the prior art. The method is to connect a reactance element to a ladder SAW filter, which is formed by coupling SAW resonators in a ladder shape, at an arm of the resonators coupled in parallel. The method actually can achieve substantial attenuation in a certain band-zone by widening the band to be attenuated; however, this method cannot substantially improve the needed steep filter characteristics. Therefore, since PCS, the standard of cellular phones in the U.S., specifies its fractional cross-band {((upper end frequency of the cross-band)−(lower end frequency thereof))/(center frequency thereof)} at 0.01, the pass-band is very close to the rejection band. In such a case, the foregoing method cannot practically attenuate signals in the rejection band close to the pass-band.